Radio frequency (RF) transmission is used in a wide range of communications applications. Without limitation, these applications include terrestrial trunked radio (TETRA), personal communications system (PCS), 3G, advanced mobile phone system (AMPS), broadcast, code division multiple access (CDMA), digital cellular system (DCS), government, general packet radio service (GPRS), global system for mobile communications (GSM), industrial, microwave, military, paging, public safety, rail, ultra-high frequency (UHF), utilities, very high frequency (VHF) and wideband CDMA (W-CDMA). Many of these systems involve the use of fixed transmitters and/or receivers, which may serve large numbers of users and operate at relatively high power levels. In some applications, such as public safety, high availability of the transmission system is essential. Accordingly, for commercial, technical and safety reasons it is desirable to incorporate appropriate monitoring devices within these systems, to enable changes in performance and/or failures of equipment to be identified and rectified quickly and safely.
While most modern high-power transmitters include internal voltage standing wave ratio (VSWR) monitoring and protection, these facilities are not generally suitable for ongoing monitoring of the health of the transmission system which might enable changes in performance to be detected early when they can be prevented from developing into more serious faults. It is therefore generally desirable to incorporate external monitoring devices, such as VSWR meters, at appropriate points within an RF transmission system. For example, monitoring devices may be installed along cables and at antennas.
Prior art monitoring devices such as, for example, the ACM series of antenna and cable monitors from Bird Technologies Group (www.birdtechnologies.com) provide many of these features. However, such prior art devices monitor the transmitted RF signal in its entirety, and are not capable of monitoring individual frequency channels within the signal. Accordingly, they are unable to detect changes in the performance of individual channels within the transmitted RF spectrum. Furthermore, a separate monitoring apparatus is required for each individual cable or antenna at a transmitter site, even where these cover different frequency bands. This results in a relatively high expense of installing a complete monitoring solution at a particular transmitter site.